Lower blankholder for sheet-forming mechanical presses

ABSTRACT

A lower blankholder for mechanical sheet-forming presses. The blankholder has at least one pressure cylinder and is provided with a device for the retaining of said blankholder in its bottom dead center position. An inner space of a working cylinder below a double-acting piston linked with a blankholder slide is connected with a reservoir filled by a pressure liquid which is under the pressure of a gas, while an inner space of said pressure cylinder above said double-acting piston is connected with a low pressure liquid reservoir by conduit means including a shut-off valve which may be closed at the bottom dead center of said slide for a predetermined time interval.

United States Patent Sekanina et al. Oct. 28, 1975 LOWER BLANKHOLDER FOR 2,694,567 11/1954 Hoenig 267/119 SHEET F( MECHANICAL PRESSES 3,468,336 9/1969 Barrett et a1 .1 267/119 [75] Inventors: Milan Sekanina, Brno; Miloslav Kolbert, siluvky, both of Primary ExammerLwell A. Larson Czechoslovakia 5 AB RA [73] Assrgnee: Vyzkumny ustav tvarecich stroju a 7] ST CT h l i {val-mi, Brno A lower blankholder for mechanical sheet-forming C h l ki presses. The blankholder has at least one pressure cylinder and is provided with a device for the retaining of [22] Ffled' June 1974 said blankholder in its bottom dead center position. [21] Appl. No.: 476,855 An inner space of a working cylinder below a doubleacting piston linked with a blankholder slide is connected with a reservoir filled by a pressure liquid [52] US. Cl 72/465; 267/119 which is under the pressure of a gas, while an inner [51] hit. Cl. B211) 24/02 space of i pressure cylinder above Said double [58] Fleld of Search 267/119; 72/417, 465 acting piston is connected with a low pressure q i reservoir by conduit means including a shut-off valve [56] References C'ted which may be closed at the bottom dead center of said UNITED TATE PATE T slide for a predetermined time interval. 1,846,439 2/1932 Rode et a1. 267/119 2,584,339 2 1952 Georgeff 267/119 2 2 D'awmg Fgures U.S., Patent Oct. 28, 1975 Sheet 1 of 2 3,914,978

US. Patent Oct. 28, 1975 Sheet 2 of2 3,914,978

LOWER BLANKHOLDER FOR SHEET-FORMING MECHANICAL PRESSES The object of the present invention is a lower blankholder with retaining device for mechanical presses, particularly for large single-acting presses for car body production.

Lower blankholders with retaining device are used for the holding of edges of a metal sheet during the stamping operation and for the ejection of the stamped pieces; a retaining device is used if stamped pieces are to be ejected after a predetermined delay. Lower blankholders used up to this time may be divided into two basic groups, i.e. pneumatic and air-hydraulic ones respectively; both these groups have the same drawback; rapid corrosion of the parts subjected to the pressurized air, since this air often contains large amounts of absorbed water.

Pneumatic blankholders impart the necessary pressure against the action of a ram by means of a system of pneumatic cylinders arranged one above another and during the operation of a blankholder, i.e. during the working stroke of a ram, the pistons of said pneumatic cylinders blast the pressurized air into a large pressure vessel, wherein the dimensions of said vessel prevent an increase of pressure above the predetermined level when a total volume is decreased. The retaining device comprises a supplementary hydraulic double-acting cylinder wherein the piston of said cylinder is connected with a piston rod which is common to a system of pneumatic cylinders. At bottom dead center the oil fill of a cylinder is closed by a remote controlled valve for a time interval corresponding to the desired delay, which may be set by an adjustable cam. The corrosion of the inner surfaces of the pressure cylinders and pistons due to the pressurized air results not only in their decreased functional reliability, but even in increased wear of sealing collars which must be often replaced; the replacement of said sealing collars is difficult and laborious. The consequences of corrosion are most severe if the blank-holder is not in use for extended time periods. Another drawback of the embodimerit described hereinbefore is in its complexity and high investment costs, due to its structural height and the depth of the foundations below the machine; deep foundations are necessary because the system of pressure cylinders is arranged in a vertical position. Further, the height of projecting pneumatic blankholders hinders the use of belt conveyors for scrap removal. Still another drawback is an excessive consumption of pressurized air.

Air-hydraulic blankholders impart the necessary pressure against the action of a ram by means of throttling oil by a valve, said oil being forced against a valve seat by a pneumatic cylinder piston. Throttled oil is fed into an oil reservoir from which it is forced out by the pressure of the air above its level during the return stroke. The backward flow of oil is stopped at the bottom dead center, closing a throttle valve for a time in terval necessary for the retaining of a blankholder, said time interval being set by an adjustable cam. A pulse for opening the valve is obtained from an end switch. The main drawback of air-hydraulic blankholders is the hard impact of the ram and the non-uniformity of the work, together with vibrations, resulting in an adverse influence on product quality. in the mechanized load ing and unloading of work, or in automatic production lines, the adverse influence of these blankholders is manifested by a decreased total output of the whole production line, due to the heating of oil throttled by the valve. Still another drawback is a too low ejection force, which reaches about 1/6 of the holding force under optimum circumstances. Therefore air-hydraulic blankholders are now being replaced by purely pneumatic ones, as described hereinbefore, and are not now used in the design of mechanized and automated production lines.

The drawbacks described above are mitigated by a lower blankholder according to this invention. Such blankholder comprises at least one pressure cylinder; an inner space of said cylinder below a double-acting piston linked with the blankholder slide is connected with a reservoir filled by pressure liquid which is under the pressure of gas; an inner space of said pressure cylinder above said double acting piston is connected with a low pressure liquid reservoir by means of a pipe including a valve which may be closed at the bottom dead center of a slide for a predetermined time interval; the upper portion of said slide is linked with a piston rod by a spherical head, a support, and a spring.

The design of a lower blankholder according to the present invention with liquid filling prevents the corrosion of parts of the working mechanism due to the contact with pressurized air, this being the case in pneumatic blankholders. Such arrangement results in a considerable increase in the service life of the working cylinders, pistons, sealing collars, and other parts so that maintenance costs are reduced, apart from the lower consumption of pressurized air. The shocks and vibrations, characterizing air-hydraulic blankholders, are not present in hydraulic systems, since the liquid is cushioned by a pressurized gas, and thus the seating and the whole operation are soft and elastic as in pneumatic systems.

The main benefit of this invention, however, is in the simplicity of design resulting in a considerable decrease of investment costs in comparison with the known types of blankholders. Due to the reduction of the overall dimensions of the working cylinders, the required space in the table cavity is decreased as well, so that the blankholder may be used even for wide presses with narrow tables. The holding force may be adjusted throughout an almost unlimited range, in contradistinction to pneumatic systems wherein the diameter of their cylinders is dependent on the desired force. As to the total structural height, it may be decreased to /3 or A1 of the height of pneumatic systems and thus auxiliary automation systems may be used.

An illustrative embodiment ofblankholder according to the present invention is described in detail hereinafter, taken with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic view partially in front elevation and partially in section of one of the two blankholding units comprising the whole blankholder; and

FIG. 2 is a fluid circuit diagram of the blankholder according to FIG. 1.

A table ll of a press (not shown in the drawing) is provided with a cavity 2 for blankholding units (FIG. 1), and with supporting frames 3 fastened to its underside by means of bolts 4 and nuts 5 Attached to the supporting frames 3 from above are the respective working cylinders 6 clamped between said frames 3 and upper lids 7 by tie bolts 8 with nuts 5. Sliding in each of said working cylinders 6 is a double-acting piston 10 provided with sealing collars l1 and linked with a piston rod 12. The lower portion of said piston rod is guided in a sliding bushing 16 of said supporting frame 3, while its upper portion is guided in a similar sliding bushing of said upper lid 7. The inner space of said working cylinder 6 is sealed by collars 17, secured against loosening by means of screwed retaining rings 18. The working cylinder 6 is provided with an upper opening 19 and a lower opening 20 for the intake and withdrawal, respectively, of a pressure liquid.

Telescoped over the upper portion of the piston rod 12 is a spherical head 21 which is pushed against the face of said piston rod 12 by an expanding bushing 22; said bushing 22 is inserted into an opening in the upper part of a slide 27 and held by a locking nut 23. Bearing against the spherical head 21 from above by its spherical recess is a support 24 secured against side movement by screws 25. Inserted between the upper portion of slide 27 and said support 24 is a ring-shaped rubber spring 28. Inserted into an axial groove 29 provided within an upper portion of the piston rod 12 is a key 30, which is engaged with the corresponding groove (not shown) in an opening in upper portion of a slide 27. For this purpose a wall of said expanding bushing 22 is provided with an opening (not shown) into which said key 30 extends. A slide 27 is guided by gibs 31 in the guide 26 in the cavity in table 1. The spaces above pistons 10 of both working cylinders 6 are interconnected by piping 32 connected with a pipe 33 leading to the shut-off valve 34 and further to a reservoir 13 of low pressure liquid (FIG. 2). Said shut-off valve 34 is controlled by means of an auxiliary piston 39 of a pneumatic valve 40, both the upper and lower spaces of said valve being connected with an air distributor 41 supplied by pressurized air directly from the air distribution system and controlled by an electromagnet energized by electrical pulses from the control cam box. The degree of opening of the shut-off valve may be adjusted by a locking nut 42 on the lower extended portion of shut-off valve cone.

The spaces below pistons 10 are interconnected by a piping 35 and are connected with a common pipe 14 leading to a nitrogen reservoir 36 with a resilient bag. The lower portion of a piston rod 12 is provided with a thread for receiving a regulating nut 37 of a mechanism (not shown) for stroke adjustment, which is mounted in a box 38.

A device according to this invention may be operated either with a predetermined delay at bottom dead center or with a smooth working cycle, as necessary. During the working stroke a ram of the press acts on a slide 27 by means of driving pins and a common plate forcing said slide downwards together with the doubleacting piston 10 so that the pressure liquid is forced out of inner spaces of working cylinders below piston 10 through pipe 14 to the nitrogen reservoir 35. The larger the quantity of pressure liquid coming from working cylinder 6 the smaller is the volume of nitrogen in the resilient bag so that its pressure increases. At the same time, a space above said pistons 10 is filled by a lowpressure liquid flowing from a reservoir 13 through the shut-off valve 34 which is normally open due to the air pressure acting on the lower area of an auxiliary piston 39. If the device is adjusted for a smooth working cycle,

i.e. without delay in the bottom dead center, a slide 27 is returned together with a ram. A return stroke is effected by the expansion of pressurized nitrogen in a resilient reservoir bag 36 of nitrogen, resulting in the flow of liquid back to the inner spaces of working cylinders 6 below the double-acting pistons 10. Due to the movement of said pistons the low-pressure liquid is forced out of the space above double-acting pistons 10 through opened shut-off valve 34 back to a reservoir 13.

If the device is to work with delay, an electric pulse from the control cam box turns on a magnet of an air distributor 41 before the bottom dead center is reached, and thus the space below an auxiliary piston 38 is opened and pressurized air is discharged into the atmosphere. At the same time, pressurized air is admitted into the space above an auxiliary piston 39 and said piston closes the shut-off valve 34. Therefore the lowpressure liquid may not leave the space above pistons 10 of the working cylinders 6, thus preventing the upward movement of the slide 27 until the shut-off valve 34 is opened, i.e. until the electromagnet of an air distributor 41 is deenergized. By adjusting the locking nut 42 the speed of the return stroke of the slide 27 may be controlled.

Lowering of a blankholder to the lowest position which is necessary e.g. before the run-out of a table, after ending of stamping operation, etc., is done by the draining of liquid from the nitrogen reservoir 36 through a regulated non-retum or check valve 43 into a reservoir (not shown) provided with a clean charging pump with a separate electric motor. For adjustment of the holding pressure and maintaining its value at a constant value, a pressure switch 44 is used for controlling the motor of said charging pump.

Although the invention is illustrated and described with reference to one preferred embodiment thereof, it is to be expressly understood that it is in no way limited to the disclosure of such a preferred embodiment, but is capable of numerous modifications within the scope of the appended claims.

We claim:

1. In a mechanical press for forming sheet metal having a reciprocable lower blankholder, at least one pressure cylinder, and a device for retaining said blankholder in its bottom dead center position, the improvement which comprises a blankholder slide, means providing an inner space of a working cylinder below a double-acting piston, means linking the piston with the blankholder slide, a reservoir filled by pressure liquid which is under the pressure of gas, means connecting said inner space with said reservoir, conduit means connecting an inner space of said pressure cylinder above said double-acting piston with a second, low pressure liquid reservoir, a shut-off valve interposed in said conduit means, and means for closing the shut-off valve at the bottom dead center of said slide, retaining it closed for a predetermined time interval, and thereafter opening it.

2. A lower blankholder as claimed in claim 1, wherein the upper portion of said slide of the blankholder is linked with a piston rod by a spherical head,

a support, and a spring. 

1. In a mechanical press for forming sheet metal having a reciprocable lower blankholder, at least one pressure cylinder, and a device for retaining said blankholder in its bottom dead center position, the improvement which comprises a blankholder slide, means providing an inner space of a working cylinder below a double-acting piston, means linking the piston with the blankholder slide, a reservoir filled by pressure liquid which is under the pressure of gas, means connecting said inner space with said reservoir, conduit means connecting an inner space of said pressure cylinder above said double-acting piston with a second, low pressure liquid reservoir, a shut-off valve interposed in said conduit means, and means for closing the shut-off valve at the bottom dead center of said slide, retaining it closed for a predetermined time interval, and thereafter opening it.
 2. A lower blankholder as claimed in claim 1, wherein the upper portion of said slide of the blankholder is linked with a piston rod by a spherical head, a support, and a spring. 